The present invention relates to a process for compensating temperature variations in devices incorporating two elastic surface wave oscillators, whose oscillation frequencies are mixed in such a way as to supply the difference between these frequencies.
Elastic surface waves which can propagate in a piezoelectric substrate are used in numerous applications and in particular, in conjunction with interdigitated transducers, for producing delay lines or resonators looped by means of amplifiers in order to provide oscillators operating in a range of very high frequencies (typically 100 MHz). The propagation rate of these waves in the substrate is liable to vary with different physical parameters, such as acceleration and pressure, so that such oscillators are more particularly used in accelerometers and force and pressure transducers. So as to reduce the frequency of the output signal in such a way that it can be more easily used, the transducers generally incorporate two oscillators, whose output signals have very similar frequencies and for example vary in the opposite direction to the parameter to be measured. These signals are mixed so as to supply an output signal, whose frequency is equal to the difference between the two oscillation frequencies and is therefore very small (typically 100 kHz). It varies with the parameter to be measured with a sensitivity equal to the algebraic sum of that of each oscillator considered separately in a quasi-linear manner, at least in a predetermined range of values.
Besides the parameter to be measured, the oscillation frequencies vary in a sensitive manner with the temperature. A very reduced temperature variation can be obtained through using two oscillators and forming the difference between the oscillation frequencies, whose variations as a function of the temperature have the same signs and similar values. However, possible dissymetries of the device and technological imperfections often make it impossible to sufficiently reduce this variation, particularly when there are very large temperature variation ranges and it is not always possible to place the device in a thermostatically controlled enclosure. This leads to systematic errors in the measurement of the parameter to be measured and these errors can reach the same order of magnitude as the parameter itself.